Architecture and apparatus for image generation

ABSTRACT

A system for image generation comprising a plurality of renderers, each having a geometry processor and a rasterizer, that operate in parallel to compute pixel values for a set of primitive objects that comprise the image to be rendered. The geometry processor transforms graphics primitive objects from their native object coordinates to screen coordinates. The rasterizer consists of an array of enhanced memory devices having a processor and memory for each pixel in a region of a screen. The processors and their associated memories operate in SIMD fashion on screen space primitive descriptions to compute and store pixel values for an entire such region. The enhanced memory devices further comprise compositors for combining their pixel values, for example, based on a visibility test, with those from a corresponding memory device of another rasterizer. The image generation system may further comprise shaders, which compute pixel colors based on pixel values computed in the plurality of renderers, and video frame buffers, which store these pixel colors and refresh a raster display device. The shaders and frame buffers consist of a renderer with additional memory devices that store texture and image data. Also disclosed are enhanced memory devices and rasterizers for use in the present image generation system.

FIELD OF THE INVENTION

The present invention relates to image generation systems in general andin particular to image generation systems using image compositiontechniques. The present invention further relates to memory structuresand hardware designs for implementing image generation systems usingimage composition techniques.

BACKGROUND OF THE INVENTION

Graphics applications such as flight and vehicle simulation,computer-aided design, scientific visualization, and virtual realitydemand high-quality rendering, high polygon rates, and high frame rates.Existing commercial systems render at peak rates up to 2 millionpolygons per second (e.g., Silicon Graphics' SkyWriter andHewlett-Packard's VRX). If antialiasing or realistic shading ortexturing is required, however, their performance falls by an order ofmagnitude.

To support demanding applications, future graphics systems must generatehigh-resolution images of datasets containing hundreds of thousands ormillions of primitives, with realistic rendering techniques such asPhong shading, antialiasing, and texturing, at high frame rates (≧30 Hz)and with low latency.

Attempts to achieve high performance levels encounter two bottlenecks:inadequate floating-point performance for geometry processing andinsufficient memory bandwidth to the frame buffer. For example, torender a scene with 100,000 polygons updated at 30 Hz, geometryprocessing requires approximately 350 million floating-point operationsper second, and rasterization requires approximately 750 million integeroperations and 450 million frame-buffer accesses. Parallel solutions aremandatory.

Some current systems use pixel-parallelism for rasterization;frame-buffer memory is divided into several interleaved partitions, eachwith its own rasterization processor. This multiplies the effectiveframe-buffer bandwidth by the number of partitions, but does not reducethe number of primitives each processor must handle, since mostprimitives contribute to most partitions. Because of this limitation,and the bandwidth limitations of commercial VRAMs, this approach doesnot scale much beyond today's rates of a few million polygons persecond.

Accordingly, it is an objective of the present invention to provide ahigh performance image generation system.

It is a further objective of the image generation system according tothe present invention to support a wide variety of rendering algorithmsand primitive types, from Gouraud-shaded polygons, to Phong-shadedvolume data, to directly rendered Constructive-Solid-Geometry objects.

It is a further objective of the present invention to provide anarchitecture which is suitable for a variety of image generationfunctions including rasterizing/rendering, shading, texturizing andimage buffering.

It is yet another objective of the present invention to provide an imagegeneration system which is scalable over a wide variety of performancelevels and to arbitrarily high levels by combining an arbitrary numberof renderers. It is also an objective of the present invention toprovide for a simplified programming model without any need forprimitive redistribution. An additional objective of the presentinvention is to provide a high bandwidth image composition networksuitable for use with antialiasing algorithms.

SUMMARY OF THE INVENTION

A first aspect of the present invention comprises an image generationsystem, comprising a primitive processing means for generating primitivescreen data and a plurality of rasterizers associated with the primitiveprocessing means for computing pixel values from the primitive screendata. The rasterizer comprises an enhanced memory device correspondingto a selected set of screen coordinates. The enhanced memory device hasfor each of the selected screen coordinates processing means forcomputing pixel values to provide a computed pixel value, storage meansassociated with each of the processing means for storing data, and acompositor buffer associated with each of the processing means forstoring the computed pixel value. The enhanced memory device also hasinput means for receiving computed pixel values from a correspondingenhanced memory device associated with a different one of the pluralityof geometry processors, compositor means for compositing the storedcomputed pixel value and the pixel value received by the input means todetermine a composited pixel value, and output means operably associatedwith the compositor means for outputting the composited pixel value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one embodiment of the imagegeneration system of the present invention.

FIG. 2 is a block diagram illustrating a second embodiment of the imagegeneration system of the present invention including shader boards.

FIG. 3 is a block diagram illustrating an embodiment of a rendereraccording to the present invention.

FIG. 4a is a block diagram illustrating an embodiment of a rendereraccording to the present invention.

FIG. 4b is a block diagram illustrating an embodiment of a shaderaccording to the present invention.

FIG. 4c is a block diagram illustrating an embodiment of a frame bufferaccording to the present invention.

FIG. 5 is a block diagram illustrating an embodiment of an enhancedmemory device according to the present invention.

FIG. 6 is a pictorial diagram illustrating an embodiment of the imagegeneration system of the present invention including shader boards inwhich the host processor is connected to each renderer and shader over aseparate fast serial link.

FIG. 7 is a block diagram illustrating the steps for rendering polygonsand the system components on which they are executed for an embodimentof the image generation system of the present invention.

FIG. 8 is a block diagram illustrating an embodiment of the controllogic for an image composition network according to the presentinvention.

FIG. 9 is a block diagram illustrating an embodiment of the control flowof an image composition network according to the present inventionshowing ready and go chains with master and slave controllers at variousstages during a transfer cycle.

FIG. 10 is a block diagram illustrating an embodiment of a command queueproviding communication buffering between a geometry processor and arasterizer according to the present invention.

FIG. 11 is a block diagram illustrating an embodiment of a rasterizeraccording to the present invention.

FIG. 12 is a block diagram illustrating an embodiment of an imagegeneration controller according to the present invention.

FIG. 13 is a block diagram illustrating an embodiment of the connectionbetween the texture ASICs and the enhanced memory devices according tothe present invention.

FIG. 14 is a block diagram illustrating an embodiment of a texture ASICchip according to the present invention.

FIG. 15 is a block diagram illustrating an embodiment of a shader boardmodule according to the present invention.

FIG. 16 is a block diagram illustrating an embodiment of one module of avideo board rasterizer according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The image generation system of the present invention exploits theadvantages of image-composition to provide for real-time 3D graphicsalgorithms and applications. The image generation system of the presentinvention may be described with reference to FIGS. 1 through 16 but isnot limited to the embodiments shown in those figures. The presentinvention will first be described generally and then a particularembodiment will be described for illustrative purposes.

As seen in FIG. 1, at its highest level the image generation system ofthe present invention is comprised of a plurality of renderers 10 actingin parallel to produce a final image. The renderers 10 receiveprimitives of a screen image from a host processor 20 over a hostinterface 25. Pixel values are then determined by the renderers 10 andthe visibility of a particular pixel calculated by a given rendererdetermined through a compositing process and stored in the frame buffer30 for display on the video display 40. The linear array of renderersresults in the final image being produced at the output of the lastrenderer.

The primitives received by the renderers are geometric shapes which maybe combined to create an image. Primitives may contain shape, depth,color, shading and other information about the image known to those ofskill in this art. The primitives may be distributed throughout theplurality of renderers 10 in any of a number of schemes known to thoseof skill in this art. The final image is then created by puttingtogether the pixel values calculated by the plurality of renderers.Image composition techniques are especially suited for use with thepresent image generation system as a means for recombining the pixelvalues from the plurality of renderers.

When image composition is used, primitives for the entire screen or asegment of the screen are distributed over the plurality of processors.Rather than having a single renderer calculate pixel values for allprimitives for a portion of the screen, each of the renderers onlycalculates pixel values for a portion of the primitives for a region ofthe screen. A region of the screen may encompass the entire screen or aportion thereof. Therefore, all of the renderers calculate pixel valuesfor the same portion of the screen and then these pixel values from eachof the renderers are combined to produce a final image for that portionof the screen. The regions of the screen are processed sequentiallyuntil the final image is produced. This combination of pixel values fora region of the screen from the plurality of renderers is referred to ascompositing.

As shown in FIG. 2, the image generation system may further includeshaders 15 which provide for texturing and shading of the image aftercomposition by the renderers 10 and before storage in the frame buffer30. The shaders may also be used to perform antialiasing operations. Theshaders receive shading instructions from the host processor 20 and thecomposited pixel values from the last renderer in the linear array ofrenderers 10. Deferred shading algorithms, such as Phong shading andprocedural and image-based textures, are implemented on separatehardware shaders 15 that reside just ahead of the frame buffer 30.Regions of pixels, containing attributes such as intrinsic color,surface normals, and texture coordinates are rasterized and compositedalong the image composition network 35 of the renderers 10 and loadedinto the shaders 15 from the image composition network 35. The imagecomposition network 35 may optionally be structured as a ring network toallow any board to communicate with any other board. It will be apparentto one of skill in the art that other means for structuring the imagecomposition network would also provide this capability. Shaders 15operate on entire regions in parallel, to convert raw pixel attributesinto final RGB values, blend multiple samples together for antialiasing,and forward final color values to the frame buffer 30 over the imagecomposition network 35.

FIG. 3 is a block diagram of a renderer 10 according to the presentinvention. Each of the plurality of renderers may be identical. Therenderers 10 are comprised of a geometry processor 100 for receivingprimitives and translating the primitives to screen coordinates toprovide primitive screen data. This primitive screen data is thentransferred to a rasterizer 120 associated with the geometry processorfor computing pixel values from the primitive screen data.Alternatively, this primitive screen data could be provided by the hostprocessor 20 or by other primitive processing means to the renderer 10.The renderer would then not include a geometry processor 100.

The rasterizer 120 is made up of an image generation controller 110 andat least one enhanced memory device 150, but preferably a plurality ofenhanced memory devices forming an enhanced memory device array 125. Theimage generation controller 110 receives the primitive screen data andprovides it to the enhanced memory devices 150. The image generationcontroller 110 may also adjust the values received from the geometryprocessor 100 before passing the values on to the enhanced memorydevices 150 for use in antialiasing. Each of these enhanced memorydevices 150 corresponds to a selected set of screen coordinates suchthat any pixel coordinate in the region of the screen being generatedhas a corresponding enhanced memory device. Each of the enhanced memorydevices 150 has a pixel processor 151 or other processing means, foreach of the set of coordinates associated with that particular enhancedmemory device for computing pixel values. The enhanced memory deviceshave storage means 152 associated with each of the pixel processors 151for storing data for use by these pixel processors. Each of the enhancedmemory devices 150 further has a compositor buffer 153 or other storagemeans associated with each of said pixel processors for storing thecomputed pixel values for each screen coordinate. The enhanced memorydevices also have a compositor input 155 or other input means, forreceiving computed pixel values from the corresponding enhanced memorydevices associated with the other renderers and a compositor output 156or other output means, for outputting the computed pixel values. Theenhanced memory devices also have compositor circuitry 154 or othercompositing means for compositing the computed pixel value stored in thecompositor buffer 153 and the pixel value received at the compositorinput 155, and providing the composited pixel values to the compositoroutput

By adding the capability to load the values received at the compositorinput 155 into the enhanced memory device 150, the renderer 10 may beutilized for shading functions. In this case, the shader 15 wouldreceive shading information from the host processor for use by thegeometry processor 100 and rasterizer 120. As described below, furthermodifications to the basic renderer design may be made to allow fortexturing.

Of particular advantage to the image generation system of the presentinvention is the modularity of the structure of the renderers, shadersand frame buffer. The single instruction multiple data (SIMD) rasterizer120 used in the renderer 10 is an ideal processor for deferred shading,because shading calculations can be performed for all pixelssimultaneously. Therefore, the shaders 15 can simply be designatedrenderers, with a slight enhancement of the compositor circuitry 154 onthe enhanced memory devices to allow bidirectional data transfersbetween the image composition network 35 and the enhanced memory devices150. Shaders can be augmented with additional hardware to allow them tocompute image-based textures in addition to procedural textures.

As seen in FIG. 4, the shader 15, shown in FIG. 4b, and frame buffer 30,shown in FIG. 4c, are substantially the same as the renderer 10, shownin FIG. 4a. As seen in FIG. 4b, the addition of a local port 160 to theenhanced memory devices 150 allows for texturing as described above. Alocal buffer 161 stores data for access by the pixel processors 151 andthe local port 160. The local port 160 provides access to local externalmemory 165 or other external memory means which may be used for storingtexturing information. Thus, by the addition of the local port 160, thelocal buffer 161 and the local external memory 165, the renderer 10 maybe utilized as a shader 15 and perform texturing algorithms.

Similarly, through the addition of the local buffer 161, the local port160 and the local external memory 165, all that need be added to theshader 15 to operate as a frame buffer 30 is the inclusion of videocircuitry 170. As shown in FIG. 4c, as a frame buffer 30, the localexternal memory 165 acts as the frame buffer memory and the videocircuitry 170 reads the pixel image data from the memory. In the framebuffer 30, the local memory 165 is, in one embodiment, a separatedouble-buffered VRAM frame buffer.

In one embodiment of the present invention, the primitives received fromthe host processor 20 are transformed by the geometry processor intoprimitive screen data represented as coefficients of mathematicalexpressions f(x,y) representing the primitive. In particular, theprimitives may be linearized into a series of equations of the formf(x,y)=Ax+By+C, where x and y are screen pixel coordinates and A, B andC are coefficients which define a plane contained in a particularprimitive. The use of linear expressions of the above form to renderprimitives is described in U.S. Pat. No. 4,590,465 at column 4, line 47through column 6, line 8, the disclosure of which is incorporated hereinby reference as if set forth fully. While linearization has beendescribed, other mathematical expressions such as quadratic, or otherpolynomial representations in screen coordinates may be utilized.

In one embodiment of the present invention, the geometry processor 100provides the A, B and C coefficients to the enhanced memory devices 150through the image generation controller 110. The image generationcontroller 110 controls the enhanced memory device array 125. Itconverts floating-point A, B, and C coefficients into byte-serial,fixed-point form; it sequences enhanced memory device operations bybroadcasting command information such as primitive data, control, andaddress information to the enhanced memory device array 125; and itcontrols the compositor ports 157 on the enhanced memory devices 150.

The image generation controller 110 may also be used for anti-aliasingthrough the use of super-sampling. Super-sampling involves calculatingthe pixel values at sub-pixel locations surrounding a pixel and thencalculating the actual pixel value for a given screen coordinate bycombination of these sub-pixel values. The image generation controller110 can modify the coefficients to reflect the sub-pixel coordinates andthen retransmit these coefficients to the enhanced memory devices 150without obtaining additional coefficients from the geometry processor100. The image generation controller 110 contains a subpixel offsetregister that allows for the multiple samples of the supersamplingfilter kernel to be computed from the same set of rasterizationcommands. This allows for increased system performance whensupersampling, because additional samples are rasterized withoutincreasing the load on the geometry processor.

FIG. 5 is a block diagram of a particular embodiment of the enhancedmemory device 150 which is utilized in the image generation system ofthe present invention. It will be appreciated by those of skill in theart that the enhanced memory device of the present invention may befabricated on a single integrated circuit or chip using fabricationtechniques known to one of skill in the art.

As seen in FIG. 5, in this embodiment the pixel processors 151 eachshare a linear expression evaluator 200 which computes values of theprimitive screen data as the bilinear expression Ax+By+C defining aplane that represents a portion of a particular primitive at every pixelprocessor 151 in parallel. Each pixel processor 151 also has a smalllocal ALU 210 that performs arithmetic and logical operations on thesegment of local memory 220 which acts as the storage means 152associated with that pixel processor and on the local value of thebilinear expression. Operation of the pixel processors 151 is SIMD(single-instruction-multiple-data), and all processors operate on dataitems at the same address. Each pixel processor 151 includes an enableregister which qualifies writes to memory, so that a subset of theprocessors can be disabled for certain operations (e.g. painting ascan-converted polygon). Also included are the local buffer 161 forproviding information for use by pixel processors 151 and receivinginformation from and outputting information to the local port 160. Thecompositor buffer 153 stores pixel values to be composited. Thecompositor buffer 153 provides its data to the compositor circuitry 154for compositing and output. The compositor circuitry 154 receives datafrom the compositor input 155 and outputs data on the compositor output156.

As seen in FIG. 6, one embodiment of the image generation system of thepresent invention may be composed of one or more card cages, eachcontaining multiple circuit boards and connected to a host processor 20.The boards in each card cage are attached to a common backplane, whichcontains an image composition network 35 that extends to each systemboard. Each board has provisions for connecting to the host processor20. The backplane further distributes power and clocks to the systemboards. Multiple card cages can be connected by placing themside-by-side and connecting their backplanes together with specialbridge boards (not shown) and connectors.

The image generation system of the present invention contains three mainboard types:

Renderers 10, which are one-board graphics computers capable ofrendering well over one million z-buffered triangles per second

Shaders 15, which are one-board graphics computers capable of computingshading models for pixels in parallel and texturing; and

Frame buffer 30, which buffers and displays composited pixels.

In a typical application, the host processor 20 is the overall systemmaster. It loads code and data onto the various system boards, and sendsthe display primitives to the renderers 10.

The system rasterizes one region of the screen at a time. This meansthat transformed primitives must be sorted into bins for each screenregion before rasterization can take place. These bins are stored inVRAM memory 502 of the geometry processor.

After the primitives have been sorted, they are converted into imagegeneration controller instructions, and rasterized one bin at a time.The image generation controller 110 and enhanced memory device array 125rasterize the primitives for each region. If several samples arerequired for each pixel, for example with supersampling antialiasing,the scene is rasterized several times with slightly different sub-pixeloffsets for each pass. When a region has been rasterized, it is copiedinto the enhanced memory devices' compositor buffer 153, and compositedover the image-composition network 35.

Shaders 15, if they are used, intercept regions of composited pixelvalues, and compute a shading model on each pixel in the region. Whenantialiasing with several samples per pixel, successive regions sent toeach shader contain successive samples. The samples are blended togetherusing precomputed blending coefficients stored at each shader. When allof the samples have been blended, the antialiased, shaded region ofpixels is transferred over the image-composition network 35 to the framebuffer 30.

The frame buffer 30 receives shaded, antialiased pixels and stores themin the appropriate portion of its local external memory 165. When all ofthe regions for a frame have been received, it switches buffers, anddisplays the new frame.

Note that rendering occurs in a discrete fashion: one 128×128 region israsterized/shaded at a time. Each board in the system must know the typeand location of each region to handle it in the appropriate way: whetherto composite it, load it into a shader, unload it, forward it on withoutprocessing, and so forth. This is done by distributing a renderingrecipe to each board before rendering begins. The rendering recipe is alist of the regions to be transferred over the image-composition networkand the corresponding action to be taken for each region. For someboards, the rendering recipe is simple. For example, the frame buffer'srendering recipe simply states whether the region is to be stored andwhere, and at which point it should switch buffers. The rendering recipefor a shader is more complicated. It must load regions, blend otherregions in, forward some regions on without processing, and unload otherregions. FIG. 7 shows the steps for rendering polygons and the systemcomponents on which they are executed.

Host Processor. The host processor 20 may be a workstation, amultiprocessing system or some other general-purpose computer, thatserves as the system master. It loads code and data onto the varioussystem boards, samples user inputs, runs the application, and sendsprimitives to the renderers 10 and shading commands to the shaders 15.

Image-Composition Network. The image-composition network 35 may be avery wide (256-bit), high-speed special-purpose communication networkfor rapidly moving pixel data between adjacent boards. It performs twofunctions in different parts of the system: it transfers pixel databetween compositors 154 in the renderers 10 and transfers renderedpixels from the shaders 15 to the frame buffer 30. The image-compositionnetwork 35 may be implemented as 256 wires that connect adjacentbackplane slots. Compositors on each system board transmit data tocompositors on an adjacent board.

The image-composition network 35 may be implemented as an extremely highbandwidth, ring-connected unidirectional datapath that extends acrossthe entire system backplane (or backplanes--in a multiple-rack system).The backplane provides point-to-point wiring between boards, but theactive circuitry is contained on each board.

The image composition network's raw bandwidth determines the maximumscreen resolution, frame rate, number of samples per pixel, and numberof bits per pixel in the system. These parameters can be traded offagainst each other, but their product must be less than the totalbandwidth.

The image composition network is composed of two parts: a data path,which carries pixel data from board to board, and a control path, whichsynchronizes and sequences transfers over the network. The imagecomposition network control path is governed by the ready/go controllerdescribed below or other appropriate image composition network controlmeans for synchronizing the transfer of data. We now describe oneembodiment of these parts in more detail.

The datapath is a 256-bit wide point-to-point communication network thatconnects each pair of adjacent boards in the system. The interconnectionwires are part of the system backplane. High-density connectors bringthe data path wires out to each board, along with associated controlsignals. All of the active circuitry for the data path is contained onindividual renderer/shader and frame-buffer boards.

Individual compositors 154 in the network are configurable under programcontrol. Compositors may operate in one of the following four modes:

Composite. Composite pixel streams from the compositor buffer 153 of theenhanced memory device 150 with pixel streams at the compositor input155. Transmit the result over the compositor output 156.

Load. Load the input stream into the enhanced memory devices.

Forward. Forward the input stream to the compositor output 156 withoutmodification.

Unload. Send data from the enhanced memory device over the compositoroutput 156. Ignore pixels arriving at the compositor input 155.

The compositing operation may be performed by several methods. In oneembodiment it is done by comparing the incoming pixel z value to thecompositor buffer pixel z value to determine which pixel is visible. Asthose of skill in the art will recognize, other methods of compositingmay be utilized with the present invention. For example, A-buffercompositing, in which pixels are represented by a variable-length listof fragments, may be used. To use A-buffer compositing, each renderer 10computes an order list of fragments for each pixel, and the compositorsinterleave the fragments of the incoming pixel with the fragments of thepixel stored in the compositor buffer, deleting fragments which can nolonger be visible.

In an embodiment using z-buffering, the Composite mode requires thatpixels have their z value first and its bits ordered frommost-significant bit (MSB) to least-significant bit (LSB). When thenearer pixel has been determined by comparing corresponding bits, thecompositor 154 sets state bits recording which pixel is in front andthat the priority decision has been made for this pixel. These two statebits determine which pixel is transmitted for the remaining z bits andall of the other data bits in the pixel.

The Composite mode is the only mode needed for renderer boards. Shaders15 need the other modes, however, to load composited regions into theirenhanced memory devices 150, unload regions that have been shaded, andto forward regions that pertain to other shaders.

Transfers over the image composition network 35 have a quantum of oneregion size. The number of bits in a pixel is variable. Once a transferbegins, however, all the pixels in a region--whatever the pixelsize--are transmitted from the source board(s) and received at thedestination board(s).

Region transfers typically perform one of two tasks. They eithercomposite regions of pixels from renderers and deposit them into shadersor transfer shaded pixels to the frame buffer. (Other operations arepossible, such as transferring a region from one renderer to another,but these are not needed in standard polygon rendering). Each regiontransfer has a particular meaning to each board in the system, and theformat of the pixel data may differ, depending on the type of transfer.Each board must know the purpose of the transfer to configure itselfcorrectly. The schedule of transfers for an individual board is called arendering recipe. The rendering recipes on each board are thedistributed controllers for the parallel system.

Transfer operations must be synchronized with great precision and lowoverhead to make maximum use of the image composition network 35. When atransfer begins and the upstream renderer begins to transmit pixels,downstream renderers must be ready to receive them at the correct clockcycle. The image composition network also must determine when all of theboards are ready to begin the next transfer. This decision must be maderapidly, since any delay affects the net bandwidth that can be achievedover the image-composition network, hence, the system's maximum speed.The control path of the image composition network implements these twofunctions. In addition, it synchronizes transfers with other componentson each board.

The ready chain and go chain are hardware interlocks for synchronizingtransfers over the image composition network 35. A ready tokenpropagates upstream from board to board, indicating when boards areready for the next transfer. A go token propagates downstream,triggering the start of a transfer.

As seen in FIG. 8, in one embodiment, the image generation controller110 performs a ready/go controller function with three inputs and threeoutputs. The XferReady input comes from the rasterizer 120. It indicateswhen the rasterizer is ready for the next transfer to begin. XferReady,ReadyIn and ReadyOut implement the ready chain; the ready token isencoded by low-to-high transitions of ReadyOut on each board. GoIn,GoOut, and XferGo implement the go chain; the go token is encoded bylow-to-high transitions of GoOut on each board.

In operation, the frame buffer 30 asserts ReadyOut when it is ready forthe next transfer to begin. The ready/go controller of each imagegeneration controller receives ReadyIn from the downstream controllerand outputs ReadyOut to the upstream controller. It asserts ReadyOutafter ReadyIn and XferReady have both been asserted. The transfer beginswhen ready reaches the upstream renderer. Thus, each board can delay thetransfer until it is ready to begin.

Go propagates through the image composition network in the oppositedirection of ready. The upstream image generation controller assertsXferGo and GoOut to begin a transfer. When each image generationcontroller receives GoIn, it asserts XferGo to the board and GoOut tothe downstream controller. The boards cannot veto go in the same mannerthey can veto ready; they lost the privilege when they assertedXferReady. The arrival of go means that n clock cycles later (n isdetermined by the startup delay of the output sequencing circuitry),pixels will flow over the composition network. When the transfer hascompleted, the image generation controller deasserts XferGo and GoOut,the frame buffer 30 (when it is ready) asserts ReadyOut, and the nexttransfer cycle begins.

The ready/go controller on the upstream renderer has a slightlydifferent function. Rather than passing ready upstream and godownstream, it simply asserts GoOut after it receives ReadyIn andXferReady. Since this controller determines when a transfer actuallybegins, it is called the master controller. The controller on each boardcan function as either a slave or master. FIG. 9 shows the ready and gochains with master and slave controllers at various stages during atransfer cycle.

Note that all of the signals in the ready and go chains (exceptXferReady) operate at the image composition clock speed to keep up withthe image-composition network data path and to reduce overhead. Even so,the overhead from the ready and go chains can be noticeable in largesystems.

The XferGo signal from the control path indicates the beginning of atransfer. This signal may arrive at any clock cycle after XferReady hasbeen asserted. The compositor sequencer configures and sequences thecompositors 154 and the compositor ports 157 so that they are ready tocomposite, load, or unload pixels n clock cycles after XferGo isasserted,

The compositor sequencer has two parts: a configuration register and atimer. The configuration register stores the control bits that determinethe compositor's operating mode (Composite, Load//Forward, or Unload)and the compositor port's transfer mode (read or write).

The timer keeps track of the number of pixels that have been compositedand enables and disables the compositor port 157 at the appropriatetime. It is preset before the transfer begins by the image generationcontroller 110 to the appropriate pixel length. After XferGo isasserted, it asserts the XferEnab signal to the enhanced memory devicecompositor port 157 for the precise length of time required to transferall of the pixels in one region.

Renderer/Shader. A single board type may be used to implement bothrenderer and shader functions. A renderer/shader board may implement anentire z-buffer rendering engine capable of rendering well over 1million z-buffered triangles per second and shading 128×128 pixels inparallel. The image generation system of the present invention can beconfigured with any number of renderer/shaders. Renderers 10 areresponsible for transforming and rasterizing primitives, and compositingthe rendered pixels with pixels from other renderers. Shaders 15 receiverendered pixels from the image-composition network, buffer them,evaluate a shading model for all of the pixels in a 128×128-pixel regionin parallel, and forward the pixels to the frame buffer 30. Arenderer/shader board has two main parts: a geometry processor 100, anda rasterizer 120.

Geometry Processor. The geometry processor includes a processing means504 which may be a fast floating-point processor. It may contain its ownlocal memory and has access to the host interface 130 and the rasterizer120. The geometry processor 100 is responsible for transformingprimitives from object coordinates into screen coordinates, computinginstructions to be passed to the rasterizer, and sorting these into binscorresponding to individual screen regions. The geometry processor'smemory 502 in one embodiment uses video RAM (VRAM) rather thanconventional DRAM. The VRAMs' random ports are connected to the memorybus. The VRAM's serial ports are connected to the image generationcontroller's input port 440. The VRAMs, together with a DMA controller510, form an input interface to the image generation controller 110.

As shown in FIG. 10, a command queue, preferably a FIFO 500 providescommunication and buffering between the geometry processor and therasterizer. The FIFO 500 stores pointers to blocks of data in thegeometry processor's VRAM memory and the length of each block.

A DMA controller 510 reads pointers from the FIFO 500 and fetches thedata from the VRAMs' serial ports. The serial ports of all of the VRAMbanks are connected together to a common 32-bit bus and are connected tothe image generation controller input port. The DMA controller 510handshakes with the image generation controller 110 to allow a 32-bitcommand word to be loaded every 66 MHz clock cycle.

Rasterizer. The Rasterizer 120 is the central part of eachrenderer/shader board. The core of the rasterizer, which is the same onevery board, along with board-specific circuitry, such as texture memoryon a shader board, or video memory on a video board are included in therasterizer. The rasterizer core contains an array of 64 enhanced memorydevices 150, an array of 32 texture ASICs 162, and an image generationcontroller 110, as shown in FIG. 11.

The enhanced memory devices 150 form an array 125 of 128×128 (16,384)byte-serial pixel processing elements, operating inSingle-Instruction/Multiple DataStream (SIMD) fashion. In typical systemoperation, these processing elements are mapped onto a 128×128 pixelregion on the screen, one processing element per pixel, with the localvalues of F(x, y) corresponding to a processing element's (x, y) pixeladdress.

The texture ASICs 162 are datapath chips, which connect the enhancedmemory devices 150 to the geometry processor bus and to optionalexternal memory 165 for texture or image storage.

The image generation controller 110 interprets instructions from thegeometry processor. It is a microcoded engine with two sequencers thatexecutes high-level instructions for the enhanced memory devices 150 andtexture ASICs 162, controlling their cycle-by-cycle operation.

In addition to the rasterizer core, a Shader Board contains DRAM memoryfor storing textures. A Video Board contains VRAM memory for storingpixels during frame scan-in and scan-out. A Renderer Board contains noextra circuitry, but the texture ASICs implement an interface betweenenhanced memory devices memory and the geometry processor bus (thisconnection is present on every board type).

Image Generation Controller. In one embodiment, the,image generationcontroller 110 processes commands from the geometry processor 100,controls and sequences the enhanced memory devices 150 and texture ASICs162, and synchronizes the rasterizer with the image-composition network35. The image generation controller may be a custom chip with separatesequencers and microcode stores for controlling the enhanced memorydevices and texture ASICs; the enhanced memory device sequencer includesa serializer to convert floating-point input coefficients into thefixed-point, byte-serial form required by the enhanced memory devices.FIG. 12 shows a block diagram of the image generation controller.

Commands for the image generation controller can have different formats.They can contain some or all of the following 5 arguments depending onthe purpose of the instruction:

I-word, mandatory, contains the instruction opcode and a few parametersfor the microcode routine. The I-word is present in every command and isthe first word in the command;

P-word, optional, contains additional parameters for the microcoderoutine; and

A, B, and C coefficients, optional, operands for the linear expressionevaluator.

The I-word and P-word are each 32-bit quantities. The A, B, and Ccoefficients may be either 32- or 64-bit integers or floating-pointnumbers. The image generation controller contains a 256-bit wide inputcommand register, holding an entire command made up of these arguments.The physical input interface to the image generation controller is32-bits wide. A stream parser 600 parses the input stream, loading eachword into the appropriate portion of the command register. The remainderof the input interface handles commands as single units, irrespective oftheir format.

The rasterizer's task consists of two parts: (1) rasterization--thecalculation (or shading) of pixel values, and (2) compositorsetup--copying pixel data to the compositor buffer section 154 ofenhanced memory device pixel memory and controlling the compositor 154.Unfortunately, the two parts must be performed as asynchronousprocesses, since multiple regions of pixel values can be buffered inpixel memory and region transfers can occur at unpredictable times (whenthe image-composition network is ready to transfer a new region).

To be able to execute these tasks asynchronously, incoming imagegeneration controller commands must be buffered in two command queues,preferably FIFOs: the RFIFO 610 buffers rendering commands, and theTFIFO 620 buffers transfer commands. Arbitration logic is needed tosynchronize the operation of the two FIFOs.

The FIFOs are wide enough to hold an entire command in each entry. A bitin the I-word of each command determines whether the command is to beloaded into the RFIFO 610 or TFIFO 620. The RFIFO 610 can hold up to 64commands and the TFIFO 620 can hold up to 1024 commands. Hardware keepstrack of the number of commands stored in the FIFOs and asserts thestatus register bit when either FIFO is nearly full. The geometryprocessor 100 detects the assertion of this status register bit toprevent overflow of Image Generation Controller FIFOs.

The FIFOs are emptied by the RT Controller 630, which keeps track of thenumber of region buffers available in pixel memory, as well as thestatus of the image-composition network 35. Initially, it reads commandsfrom the RFIFO 610. When a new image-composition network transfer needsto be (and is ready to be) initiated, the command stream from the RFIFO610 is interrupted and commands are read from the TFIFO 620. Thesecommands copy data from the storage means 152 to the compositor buffer153 and initiate a transfer. As soon as the new transfer is initiated,command stream processing reverts to the RFIFO 610.

The RT Controller 630 maintains two internal 4-bit counters: FreeBuffsand FullBuffs, indicating the number of free and full buffers of pixeldata in the enhanced memory device 150. When the image generationcontroller 110 is reset, FreeBuffs and FullBuffs are each set to 0.Software must set FreeBuffs to the number of empty pixel buffersavailable in the enhanced memory devices. These counters are used likesemaphores to determine whether commands can be safely executed fromeach of the input FIFOs. For example, when FreeBuffs=0, rasterizationmust halt; no buffer is available to hold any further results. Also, atransfer can only begin when FullBuffs>0. FreeBuffs and FullBuffs can beincremented and decremented using special control commands embedded inthe incoming command stream.

In one embodiment, the image generation controller 110 contains twosequencers, the EMC Sequencer 640 and the TAS Sequencer 650. The EMCSequencer 640 generates micro-instructions for the pixel processors 151,addresses into pixel memory 220, and ABC coefficients for the linearexpression evaluator 200; commands to the EMC Sequencer 640 are used torasterize polygons (on Renderer boards) and to perform shadingcalculations (on Shader boards). The TAS Sequencer 650 controls thetexture ASICs 162 and attached memory 165; commands to the TAS Sequencer650 are used for moving data between the pixel memory 22O on theenhanced memory devices 150 and attached memory 165 on Shader or Videoboards, or between pixel memory 220 on the enhanced memory devices 150and the geometry processor bus.

The EMC Sequencer 640 and TAS Sequencer 650 each contain local microcodememory. A bit in the I-word of each command indicates which sequencer isto execute the command. The I-word also specifies the starting microcodeaddress for the command. Each sequencer can conditionally branch, bothon internal conditions (such as the value of loop counters) and externalconditions (two condition code inputs for each sequencer). Thesequencers have no stack. They also have several external controloutputs which allow them to implement miscellaneous control functions onthe rasterizer board.

Enhanced Memory Devices. In one embodiment, the array 125 of 64 enhancedmemory devices 150 implements a 2-dimensional SIMD processor array thatcovers a 128×128-pixel region of the screen. This logical array can be"moved" to process any 128×128-pixel region of the display screen; theregion is normally aligned to 128-pixel boundaries, but it can bepositioned arbitrarily.

Each pixel is provided with its own 8-bit ALU 210, an output of thelinear-expression evaluator tree (the LEE) 200, 2048 bits of localmemory 152, a 256-bit compositor buffer 153, and a 256-bit local databuffer 161. FIG. 5 shows a logical diagram of an enhanced memory device.

Each pixel ALU 210 is a general-purpose 8-bit processor; it includes anenable register which allows operations to be performed on a subset ofthe pixels. The pixel ALU can use linear expression evaluator results orlocal memory 220 as operands and can write results back to local memory.It can also transfer data between memory and the local and compositorbuffers.

The image generation controller 110 controls the operation of theenhanced memory device array 125. Image generation controllerinstructions and coefficients are serialized and broadcast to all of theenhanced memory devices in parallel. The SIMD pixel processors executethese instructions in lock-step. The enable registers in each pixelprocessor are used to control which subset of the pixel processors areactive at any given time.

Each pixel is provided with 256+32+32 bytes of local memory. Asdiscussed above, the memory is divided into 3 partitions: a 256-bytemain partition, which is used for most computation, and two 32-bytepartitions used for external communication.

Normally, all 256+32+32 bytes of pixel memory can be accessed. However,when communication-port operations are performed, their buffer datatemporarily is unavailable. For example, after pixel data to becomposited is copied into the compositor buffer, the compositionoperation is initiated, and memory in the compositor buffer cannot beaccessed by the ALU until the composition operation is complete.Similarly, to perform a local-port operation, data is moved into thelocal buffer and the local-port operation is initiated; at this point,the local buffer must not be accessed by the ALU until the operation iscomplete.

The image-composition port and local port allow pixel data to betransferred serially to/from the enhanced memory devices to otherenhanced memory devices (for compositing) or to/from the texture ASICs(to perform texture lookups or pixel-data writes to texture or videomemory). Data from each pixel is presented serially at each port. Thenumber of bytes transferred to/from each pixel and their location in thecommunication buffer are designated by configuration commands. Theimage-composition port includes the compositor input 155 and thecompositor output 156 both of which are 4-bit ports running at 132 MHz.The local port is a 4-bit port that runs at 66 MHz.

Each enhanced memory device has an output which represents thelogical-OR of the enable registers of all pixels. These outputs arewire-anded together to form the global-enable signal (EOrH), thelogical-OR of the enable registers for the entire SIMD array. EOrH isfed into an external condition-code input of the EMC sequencer. Commandsto the EMC sequencer can test the status of EOr, and based on theresult, can conditionally execute. The status of EOr can be communicatedto the geometry processor.

Texture ASICS. The array of 32 Texture ASICS implement a data-parallelcommunication interface between pixel memory in the enhanced memorydevices, the bus of the geometry processor microprocessor, and optionaltexture or video memory. The 32 texture ASICs are divided among the 8modules, so that each module consists of eight enhanced memory devicesand four texture ASICs. The texture ASICs are connected to the enhancedmemory devices' 4-bit local ports in bit-sliced fashion, as shown inFIG. 13. The texture ASICs are also connected to a bit-sliced 32-bitbus, which spans all eight modules. This TASIC bus provides a logicalconnection between modules, so pixel data can be shared when necessary.The 4-bit interface between enhanced memory devices and the textureASICs, as well as the TASIC bus run at 66 MHz.

FIG. 14 shows a block diagram of a texture ASIC chip. Internally, it iscomposed of a dual-ported RAM for buffering andserializing/parallelizing addresses and data to/from enhanced memorydevice memory, several configurable datapaths, and a bidirectional portthat can connect directly to external memory. The clock-by-clockoperation of the texture ASIC array is controlled by the TAS Sequencerin the image generation controller, which is independent of the EMCSequencer on the image generation controller. The TAS Sequencer controlsthe enhanced memory device's local ports, however, since these interfacedirectly to the texture ASICs.

The texture ASICs' external memory ports are unused on Renderer boards;they connect to texture memory on Shader boards; they connect to videomemory on Video boards.

Renderer, Shader, Video Board Differences. The rasterizer on theRenderer Board consists simply of the Rasterizer core. This is the onlycircuitry needed for most rasterization calculations.

The rasterizer on the Shader Board contains an array of 128 4-Mbit×4DRAM (16 megabit) memory chips, attached to the TASIC memory ports. Thisadditional memory can be used to store and look up image-based textures,provide inter-pixel communication for image-warping operations, or serveas backing store for memory-intensive rendering algorithms.

The memories are divided among the eight modules, along with theenhanced memory devices and texture ASICs. The 16 DRAMs in a module arearranged as 8 banks of memory, each bank being 2K×2K bytes in size. Thetotal storage per module is 32 megabytes. The banks are controlledglobally (all eight do a memory operation at the same time), butaddresses may differ. FIG. 15 shows a block diagram of one Shader-boardmodule.

Organization of the memories on the Video Board is very similar to thatof the Shader Board; the memories are VRAMs, rather than DRAMs. Thedisplay is refreshed from the video port of the VRAMs. Since pixels arestored byte-serially, a corner-turning function must be performedbetween the VRAM serial ports and the color look-up tables and DACs.This is done using a set of programmable-logic parts, which also performthe necessary multiplexing. Transferring data to/from video memory issimilar to transferring data to/from texture memory. The enhanced memorydevices compute addresses for data items to be stored, then the textureASICs transfer the addresses and data to/from video memory.

The texture ASICs contain internal address counters for generating videorow-load addresses. These counters are initialized by the imagegeneration controller and are controlled by special inputs on eachtexture ASIC that are driven by board-level video refresh circuitry.

FIG. 16 shows a block diagram of one module of the Video Boardrasterizer. As on the Shader Board, the enhanced memory devicescalculate addresses for memory accesses, and the memories are organizedto support both fast-page and random access modes. Normally pixels wouldbe written into memory byte-serially, using a random-access cyclefollowed by fast-page mode cycles.

The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

That which is claimed is:
 1. An image generation system, comprising:(a)a plurality of geometry processors for receiving primitives andtranslating said primitives to screen coordinates to provide primitivescreen data; and, (b) a rasterizer associated with each of said geometryprocessors for computing pixel values from said primitive screen data,said rasterizer comprising an enhanced memory device, said enhancedmemory device corresponding to a selected set of screen coordinates,(i)said enhanced memory device having for each of said selected screencoordinates,processing means for computing pixel values to provide acomputed pixel value, storage means associated with each of saidprocessing means for storing data, and, a compositor buffer associatedwith each of said processing means for storing said computed pixelvalue, and (ii) said enhanced memory device further having,input meansfor receiving pixel values from a corresponding enhanced memory deviceassociated with a different one of said plurality of geometryprocessors, compositor means for compositing said stored computed pixelvalue and the pixel value received by said input means to determine acomposited pixel value; and output means operably associated with saidcompositor means for outputting said composited pixel value.
 2. Theimage generation system of claim 1 wherein said compositor means of oneof said rasterizers further comprises:loading means for storing thepixel values received from said input means for use by said processormeans.
 3. The image generation system of claim 2 wherein said one ofsaid rasterizers further comprises:(i) external memory means for storingimaging information, and (ii) wherein said enhanced memory devicefurther comprises,local buffer means operably associated with saidprocessing means for storing data from and providing data to saidprocessing means, port means for inputting data to and outputting datafrom said local buffer means to said external memory means.
 4. The imagegeneration system of claim 3 wherein said one of said rasterizersfurther comprises video generation means, operably associated with saidexternal memory means for creating a video signal.
 5. The imagegeneration system of claim 1 wherein said primitive screen datacomprises coefficients of mathematical expressions defining a primitivein screen coordinates.
 6. The image generation system of claim 1 whereinsaid rasterizer further comprises control means for supersampling saidprimitive screen data by receiving said primitive screen data from saidgeometry processor and repeatedly providing to said enhanced memorydevice, adjusted screen data for each primitive screen data received. 7.The image generation system of claim 1 wherein said enhanced memorydevice further comprises:coefficient input means for receivingcoefficients of mathematical expressions of the form f(x,y), and whereinsaid processing means comprises:(i) a mathematical expression evaluatorfor receiving the coefficients and outputting the results of evaluatingthe mathematical expressions f(x,y), and (ii) an arithmetic logic unitcorresponding to each of said selected screen coordinates for receivingthe results of said mathematical expression evaluator and performingarithmetic and logical operations on said results.
 8. The imagegeneration system of claim 1 wherein the primitive data for a selectedregion of a screen is distributed between said plurality of geometryprocessors.
 9. The image generation system of claim 1 wherein saidprimitive screen data comprises primitive screen data for a selectedregion of a screen and further comprising:image composition networkcontrol means operably associated with said compositor means forsynchronizing said compositors such that data is output by said outputmeans after all pixel values have been calculated by each of saidrasterizers for said selected region.
 10. An image generation system,comprising:(a) primitive processing means for generating primitivescreen data; and, (b) a plurality of rasterizers associated with saidprimitive processing means for computing pixel values from saidprimitive screen data, said rasterizer comprising an enhanced memorydevice, said enhanced memory device corresponding to a selected set ofscreen coordinates,(i) said enhanced memory device having for each ofsaid selected screen coordinates,processing means for computing pixelvalues to provide a computed pixel value, storage means associated witheach of said processing means for storing data, and, compositor bufferassociated with each of said processing means for storing said computedpixel value, and (ii) said enhanced memory device further having,inputmeans for receiving computed pixel values from a corresponding enhancedmemory device associated with a different one of said plurality ofgeometry processors, compositor means for compositing said storedcomputed pixel value and the pixel value received by said input means todetermine a composited pixel value; and output means operably associatedwith said compositor means for outputting said composited pixel value.11. The image generation system of claim 10 wherein said compositormeans of one of said rasterizers further comprises:loading means forstoring the pixel values received from said input means for use by saidprocessor means.
 12. The image generation system of claim 11 whereinsaid one of said rasterizers further comprises:(i) external memory meansfor storing imaging information, and (ii) wherein said enhanced memorydevice further comprises,local buffer means operably associated withsaid processing means for storing data from and providing data to saidprocessing means, port means for inputting data to and outputting datafrom said local buffer means to said external memory means.
 13. Theimage generation system of claim 12 wherein said one of said rasterizersfurther comprises video generation means, operably associated with saidexternal memory means for creating a video signal.
 14. The imagegeneration system of claim 10 wherein said primitive screen datacomprises coefficients of mathematical expressions defining a primitivein screen coordinates.
 15. The image generation system of claim 10wherein said rasterizer further comprises control means forsupersampling said primitive screen data by receiving said primitivescreen data from said geometry processor and repeatedly providing tosaid enhanced memory device adjusted screen data for each primitivescreen data received.
 16. The image generation system of claim 10wherein said enhanced memory device further comprises:coefficient inputmeans for receiving coefficients of mathematical expressions of the formf(x,y), and wherein said processing means comprises:(i) a mathematicalexpression evaluator for receiving the coefficients and outputting theresults of evaluating the mathematical expressions f(x,y), and (ii) anarithmetic logic unit corresponding to each of said selected screencoordinates for receiving the results of said mathematical expressionevaluator and performing arithmetic and logical operations on saidresults.
 17. The image generation system of claim 10 wherein theprimitive screen data for a selected region of a screen is distributedbetween said plurality of rasterizers.
 18. An image-generation system,comprising:(a) a plurality of geometry processors for receivingprimitives for a selected region of a screen and translating saidprimitives to screen coordinates to provide primitive screen data; and,(b) a rasterizer associated with each of said geometry processors forcomputing pixel values from said primitive screen data, said rasterizercomprising,image generation control means for receiving said primitivescreen data from said geometry processors and converting said primitivescreen data to command information; a plurality of enhanced memorydevices, each of said enhanced memory devices corresponding to aselected set of screen coordinates,(i) each of said enhanced memorydevices having for each of said selected screen coordinates,processingmeans for receiving said command information and computing pixel valuesto provide a computed pixel value, storage means associated with each ofsaid processing means for storing data, and, compositor bufferassociated with each of said processing means for storing said computedpixel value, and (ii) each of said enhanced memory devices furtherhaving,input means for receiving computed pixel values from acorresponding one of said enhanced memory devices associated with adifferent one of said plurality of geometry processors, compositor meansfor compositing said stored computed pixel value and the pixel valuereceived by said input means to determine a composited pixel value, andoutput means operably associated with said compositor means foroutputting said composited pixel value; and said rasterizer furthercomprising image composition network control means operably associatedwith each of said compositor means for synchronizing said compositorssuch that data is output by said output means after all pixel valueshave been calculated for said selected region.
 19. The image generationsystem of claim 18 wherein said compositor means of one of saidrasterizers further comprises:loading means for storing the pixel valuesreceived from said input means for use by said processor means.
 20. Theimage generation system of claim 19 wherein said one of said rasterizersfurther comprises:(i) external memory means for storing imaginginformation, and (ii) wherein said enhanced memory device furthercomprises,local buffer means operably associated with said processingmeans for storing data from and providing data to said processing means,port means for inputting data to and outputting data from said localbuffer means to said external memory means.
 21. The image generationsystem of claim 20 wherein said one of said rasterizers furthercomprises video generation means, operably associated with said externalmemory means for creating a video signal.
 22. The image generationsystem of claim 18 wherein said primitive screen data comprisescoefficients of mathematical expressions defining a primitive in screencoordinates.
 23. The image generation system of claim 18 wherein saidimage generation control means further comprises supersampling means forsupersampling said primitive screen data by receiving said primitivescreen data from said geometry processor and repeatedly providing tosaid enhanced memory devices adjusted screen data for each primitivescreen data received.
 24. The image generation system of claim 18wherein said processing means comprises:(i) a mathematical expressionevaluator for receiving coefficient data and outputting the results ofevaluating the mathematical expressions f(x,y), and (ii) an arithmeticlogic unit corresponding to each of said selected screen coordinates forreceiving the results of said mathematical expression evaluator andperforming arithmetic and logical operations on said results.
 25. Theimage generation system of claim 18 wherein the primitive data for aselected region of a screen is distributed between said plurality ofgeometry processors.
 26. A rasterizer for computing pixel values fromprimitive screen data in an image generation system comprising:aplurality of enhanced memory devices, each of said enhanced memorydevices corresponding to a selected set of screen coordinates,(i) eachof said enhanced memory devices having for each of said selected screencoordinates,a processing means for computing pixel values to provide acomputed pixel value, a storage means associated with each of saidprocessing means for storing data, and, a compositor buffer associatedwith each of said processing means for storing said computed pixelvalue, and (ii) each of said enhanced memory devices furtherhaving,input means for receiving computed pixel values, compositor meansfor compositing said stored computed pixel value and the pixel valuereceived by said input means to determine a composited pixel value; andoutput means operably associated with said compositor means foroutputting said composited pixel value.
 27. The rasterizer of claim 26wherein said compositor means further comprises:loading means forstoring the pixel values received from said input means for use by saidprocessor means.
 28. The rasterizer of claim 27 wherein said one of saidrasterizers further comprises:(i) external memory means for storingimaging information, and (ii) wherein said enhanced memory devicefurther comprises,local buffer means operably associated with saidprocessing means for storing data from and providing data to saidprocessing means, port means for inputting data to and outputting datafrom said local buffer means to said external memory means.
 29. Therasterizer of claim 28 further comprising video generation means,operably associated with said external memory means for creating a videosignal.
 30. The rasterizer of claim 26 wherein said primitive screendata comprises coefficients of mathematical expressions defining aprimitive in screen coordinates.
 31. The rasterizer of claim 26 whereinsaid rasterizer further comprises supersampling means for supersamplingsaid primitive screen data by repeatedly providing to said enhancedmemory devices adjusted screen data for each primitive screen datareceived.
 32. The rasterizer of claim 26 wherein said enhanced memorydevice further comprises:coefficient input means for receivingcoefficients of mathematical expressions of the form f(x,y), and whereinsaid processing means comprises:(i) a mathematical expression evaluatorfor receiving the coefficients and outputting the results of evaluatingthe mathematical expressions f(x,y), and (ii) an arithmetic logic unitcorresponding to each of said selected screen coordinates which receivethe results of said mathematical expression evaluator and performingarithmetic and logical operations on said results.
 33. The rasterizer ofclaim 26 wherein said primitive screen data comprises primitive screendata for a selected region of a screen and further comprising:imagecomposition network control means operably associated with saidcompositor means for synchronizing said compositors such that data isoutput by said output means after all pixel values have been calculatedfor said selected region.
 34. An enhanced memory chip for use in animage generation system comprising:a plurality of processing means forcomputing pixel values to provide a computed pixel value, a storagemeans associated with each of said processing means for storing data, acompositor buffer associated with each of said processing means forstoring said computed pixel value, and input means for receivingcomputed pixel values, compositor means for compositing said storedcomputed pixel value and the pixel value received by said input means todetermine a composited pixel value; and output means operably associatedwith said compositor means for outputting said composited pixel value.35. The enhanced memory chip of claim 34 wherein said compositor meansfurther comprises:loading means for storing the pixel values receivedfrom said input means for use by said processor means.
 36. The enhancedmemory chip of claim 35 further comprising:local buffer means operablyassociated with each of said processing means for storing data from andproviding data to said processing means, port means for inputting datato and outputting data from said local buffer means.
 37. The enhancedmemory chip of claim 34 wherein said compositor means furthercomprises:forwarding means for providing said received pixel value tosaid output means for outputting.
 38. The enhanced memory chip of claim34 wherein said processing means selectively read and write data fromsaid compositor buffer means.
 39. The enhanced memory chip of claim 34wherein said processing means selectively read and write data from saidcomposite buffer means.
 40. The enhanced memory chip of claim 36 whereinsaid processing means selectively read and write data from said localbuffer means.
 41. The enhanced memory chip of claim 34 furthercomprising:coefficient input means for receiving coefficients ofmathematical expressions of the form f(x,y), and wherein said pluralityof processing means comprises:(i) a mathematical expression evaluatorfor receiving the coefficients and outputting the results of evaluatingthe mathematical expressions f(x,y), and (ii) a plurality of arithmeticlogic units for receiving the results of said mathematical expressionevaluator and performing arithmetic and logical operations on saidresults.
 42. The enhanced memory chip of claim 34 further comprisingmicroinstruction input means operably associated with said arithmeticlogic units for receiving microinstructions to control the operation ofsaid arithmetic logic units.